Heat 1 tablespoon of the oil in a large skillet over medium-high heat. Season the pork with 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook until browned and cooked through, 5 to 6 minutes per side. Transfer to a plate.
Heat the remaining tablespoon of oil in the skillet over medium heat. Add the onion and cook, stirring occasionally, for 3 minutes. Add the vinegar and cook until the onion is soft, 2 to 3 minutes.
Add the escarole and 1/4 teaspoon each salt and pepper. Cook, tossing, until wilted, 2 to 3 minutes. Serve with the pork.
Tip: The assertive winter flavors of the escarole-and-onion mixture make it a great side dish for more than just pork chops. Try it with roasted turkey, beef, or meaty fish, such as salmon.